Currently, most of the electric power-generating solar cells are silicon solar cells. A conventional silicon solar cell structure has a large area p-n junction made from a p-type silicon wafer, a negative electrode that is typically on the front-side or sun-side of the cell, and a positive electrode on the back-side. It is well-known that radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate hole-electron pairs in that body. The potential difference that exists at a p-n junction causes holes and electrons to move across the junction in opposite directions and thereby gives rise to flow of an electric current that is capable of delivering power to an external circuit.
Process flow in mass production of solar cells is generally aimed at achieving maximum simplification and minimization of manufacturing costs. Electrodes in particular are made by using methods such as screen printing from a metal paste. During the formation of a silicon solar cell, an aluminum paste is generally screen printed and dried on the back-side of the silicon wafer. The wafer is then fired at a temperature above the melting point of aluminum to form an aluminum-silicon melt, subsequently, during the cooling phase, an epitaxially grown layer of silicon is formed that is doped with aluminum. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell. However, due to the lack of a high quality passivation layer, the current state-of-the-art cells still suffer from recombination of photogenerated carriers, either within the BSF layer, or at the back surface of the cell. This loss of photo-generated carriers leads to a loss in efficiency.
Furthermore, in an attempt to reduce total manufacturing cost of the silicon solar cells, thinner silicon wafers are being used. However, the use of silicon wafers thinner than 200 microns can lead to an increase in bowing of the silicon substrate, and hence the solar cell, due to the difference in coefficient of thermal expansion of the metal electrode material and the silicon substrate.
Hence, there is a need for back-side aluminum paste compositions and methods of making solar cells using the back-side aluminum paste compositions to decrease bowing and improve efficiency of the solar cells.